| Compared with the vacuum evaporation,solution-processing has a broad application prospect,due to low costing,employing spit-coating,printing,roll-to-roll to realize the production of large area.Resulting from many merits,such as excellent film-forming property,machining performance,difficult crystallization and other physical properties and structural diversification,polymers have been investigated extensively.But polymers had a polydispersity inherently so that it had an influence on the device performances.Therefore,monodisperse and well-defined oligomers as light-emitting layer can repeatable product as well as have superior chemical purity,which can be easily reached.In other words,oligomer was polymer with low molecular weight.It can be used as a model of polymer,to explore the internal relation between polymer and oligomer.Benzothiophene-S,S-dioxide unit has a low-lying lowest unoccupied molecular orbital(LUMO),high fluorescence quantum yields and electron mobility.Endowing SO to light-emitting materials can improve the injection and transportation of electrons.Constructing D-A type using electron-withdrawing SO and electron-donating unit,as the intramolecular charge transfer(CT)state inducing luminescence,exhibited high efficiency.Herein,we synthesized a series of oligomers and red,green,blue-lighting polymers taking advantage of SO in this paper.Aiming at improving the photoelectric properties,we choose SO as receptor unit and different donor units to balance the carrier.(1)Changing the number of electronic unit fluorene,We developed a series of D-A-D typed oligomers based on 3,7-and 2,8-substituted benzothiophene-S,S-dioxide,namely 37 SOFn and 28 SOFn,respectively.With the increase of the effective conjugation length,(1)the UV-vis absorption and PL spectra in solution showed a trend of red shift;(2)the optical band gap of oligfluorenes gradually narrowed,and more and more close to the polymer PFSO10.It shows that the effective length of conjugated polymer was close to nine units;(3)two-photon absorption capacity became stronger.Two-photon absorption cross section of nonamer had the maximum value.Via PL spectra in different solution and theoretical calculation,there is a strong CT state of oligomers in the solution,and had obvious intramolecular charge transfer effect.The single-layer devices based on 37 SOFn showed the maximum luminous efficiency(LEmax)2.18 cd A-1 and the single-layer devices based on 28 SOFn showed LEmax 1.35 cd A-1,which was better than that based on polymer PFSO10.After the addition PVK-layer,devices performances became better obviously.The maximum luminous efficiency(LEmax)of double-layer devices based on 37 SOFn and 28 SOFn increased to 3.39 cd A-1 and 1.94 cd A-1,respectively.The electroluminescent spectra of the devices showed excellent stable in applied driving voltage in the range of 5-10 V.(2)We synthesized the D-A-D typed novel green-lighting unit based on SO and thiophene,and further introduces into the poly(2,7-fluorene),poly(2,7-carbazole)and poly(3,6-carbazole)with varying content.Through two-photon excited fluorescence,polymers exhibited good two-photon absorption ability.And the single-layer devices based on the related polymers as emitting-layer obtained the maximum luminous efficiency of 10.8 cd A-1,the maximum external quantum efficiency of 4.43%,the driving voltage of 2.75 V,and maximum luminance of 16485 cd m-2.(3)Introducing electron-donating 9,9-di(triphenylamine)fluorene(FTPA)unit and electron-withdrawing SO unit in the poly(2,7-fluorene),a series of green and red polymers with double transmission ability were synthesized.The polymers displayed obvious solvation effects only when SO and FTPA were in the chain at the same time.As a light-emitting layer in single-layer devices,green-lighting polymers obtained the maximum luminous efficiency of 12.6 cd A-1;red-lighting polymers obtained the maximum luminous efficiency 2.9 cd A-1 with the maximum external quantum efficiency of 3.2 %. |
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